Questions and Answers

Ford Mustang Hesitation Problem Q. Hi, I have a hesitation problem in my car. I hope you can help me out on this. It is a 1985 Ford Mustang GT with a carbureted 302 and a 5 speed manual transmission. I just bought this car a month ago and have tried the basic stuff.

I checked the vacuum on the engine and it runs at 18.5" vacuum at idle. Yes it does move a little, but not more than ½" above or below. I checked the EGR valve and it was defective (no vacuum), so I replaced it. That did not help. I visually checked the vacuum lines and found one broken, that helped a little.

The car has 121,000 kilometers and the carburetor had been worked on. So after not finding any other problems, I bought a rebuilt carburetor. This also did not help. The spark plug wires are new as well as the plugs, not saying that they are fine, but they should be okay. Any place to look would be greatly appreciated.

Patrick

A. On an engine equipped with a carburetor, the first thing you should check is the fuel mixture. To do this you will need a four gas emission analyzer. Since I'm sure you don't have one you should take it to a shop that does have one. You want HC and CO as low as possible and the CO2 as high as possible.

Once the fuel mixture is adjusted correctly, and it still hesitates, the next thing to check is the distributor vacuum advance. There are two types used. One is a centrifugal advance and the other is a vacuum advance.

The centrifugal advance mechanism varies the relationship of the armature to the stator assembly. The armature is mounted to the sleeve and plate assembly which rotates in relation to the distributor shaft. The rotation is a result of the centrifugal weights moving in response to engine RPM. The movement of the weights change the initial relationship of the armature to the stator assembly by rotating the sleeve and plate assembly ahead of its static position on the distributor shaft This produces spark advance. The rate of movement of the weights is controlled by calibrated springs.

The single diaphragm assembly also changes the armature to stator relationship to give spark advance. The stator assembly position is changed by means of vacuum being applied to the diaphragm assembly. Vacuum applied to the diaphragm assembly causes the diaphragm and attached diaphragm rod to move, compressing the advance spring, which controls the rate of advance.

The movement of the diaphragm rod, which is attached to the stator assembly, makes the stator assembly move with respect to the armature. This changes the initial armature to stator relationship set during initial timing, causing spark advance. The stator assembly is mounted on the lower plate assembly which with the diaphragm assembly, is attached to the distributor base.

When the engine is at idle, disconnect and plug the vacuum advance vacuum line. Then set timing to 10° BTDC at 800 rpm. Check the engine decal for the timing specification for your particular engine. Use that specification if it's different then the one I gave you.

If you have a distributor with centrifugal advance, bring the engine to 2,500 rpm and check the timing. It should be between 10 to 14° BTDC.

This is a general procedure and specifications. Exact procedures and specification depends on which distributor and ignition control unit you have and if it is Federal or California emissions. If you don't know for sure, have the mechanic set the timing when the carburetor is adjusted.

The timing can be advanced a degree or two to account for engine wear.

If you still have the hesitation you should look at the carburetor again. I have seen many rebuilt carburetors be as bad as the ones being replaced. But I think between the carburetor and timing adjustments, you will cure the hesitation.